An image sensor is a semiconductor device that converts an optical image to an electric signal. Such image sensors may be classified as one of a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
In accordance with CMOS technology using a control circuit as a peripheral circuit and a signal processing circuit simultaneously, the CMOS image sensor prepares MOS transistors corresponding to the number of unit pixels and sequentially detects outputs of the respective unit pixels through the MOS transistors in a switching manner. The CMOS image sensor may include a photodiode and a plurality of MOS transistors, and converts light, i.e., a visible ray being emitted from the front and the back of an image sensor chip into electric signals, accordingly embodying an image.
A vertical image sensor having a vertical photodiode has been developed and is capable of achieving various colors in one unit pixel, which differs from a typical horizontal image sensor.
FIG. 1 illustrates a CMOS image sensor, which is fabricated as follows.
As illustrated in FIG. 1, at least one photodiode 2 is formed in semiconductor substrate 1. Next, interlayer dielectric 3 having a multilayer structure that includes metal lines is formed on and/or over semiconductor substrate 1 including photodiode 2. Passivation layer 4 is formed by vapor-depositing an oxide or a nitride on and/or over interlayer dielectric 3. At least one color filter is then formed on and/or over passivation layer 4 corresponding to photodiode 2. Lastly, at least one micro lens 7 is formed, which may include a planarization layer at a lower part thereof. Such an image sensor fabricating process includes forming micro lens 7 that focuses light, forming the color filter that discriminates signals of respective colors, for example red, green and blue, and forming photodiode 2 that generates electric signals by collecting electrons generated from the focused light.
Interlayer dielectric 3 of such an image sensor is relatively thicker than an interlayer dielectric of a CCD. The CMOS image sensor has a less pixel pitch owing to such a thickness difference. Therefore, the focusing performance with respect to photodiode 2 is deteriorated more seriously than in the CCD although micro lens 7 in the optimum state is applied. This is because the minimum spot size to achieve the focusing with the optimum-state micro lens 7 is proportional to a focal distance and also related to the numerical aperture (NA). The NA in a pixel of the image sensor corresponds to the pixel pitch while the focal distance corresponds to the thickness of interlayer dielectric 3 that contains the metal lines therein. Accordingly, it is required to reduce the size of the pixel and the thickness of interlayer dielectric 3 in order to obtain a focal spot.
In accordance with the CMOS image sensor structure, however, it is limited to reduce the thickness of interlayer dielectric 3 to the minimum necessary thickness. Meaning, there occurs a limited pixel pitch of about 1.75 μm which has been measured as the minimum limit pitch no more reducible. Furthermore, the structure as illustrated in FIG. 1 is limited to effectively focus the light using only micro lens 7 because of the insufficient focusing performance to photodiode 2. Another micro lens, made of an inorganic material, may be further provided in interlayer dielectric 3 in addition to micro lens 7 disposed at the upper part, to thereby overcome certain limitations. However, this considerably complicates the fabrication processes.
As illustrated in FIG. 2A, as an alternative, wave guide 8 may be further provided to serve as a path of incident light. Such a structure is formed by forming a trench by almost a pixel size at an upper part on and/or over photodiode 2 as deep as the thickness of interlayer dielectric 3. Next, wave guide 8 is formed by filling the trench with a material having greater spin-on-glass or refractivity (refraction coefficient) than interlayer dielectric 3. Therefore, the incident light can be efficiently transmitted to photodiode 2 through wave guide 8. However, etching needs to be performed to produce the wave guide, and thus, may cause damage to plasma by the photodiode, thereby increasing dark current and deteriorating the sensitivity of the image sensor. Also, the fabrication processes are complicated.